This invention relates to self-lubricating surfaces, and, more particularly, to a method for affixing a self-lubricating material to the external surface of a substratum.
Many bearings are known which have a self-lubricating surface. For instance, it is known to use low friction fabric of woven Teflon (a DuPont trademark for tetrafluoroethylene) filaments adhered to structural bearing elements to provide a low friction surface. In U.S. Pat. Nos. 3,974,009, 3,891,488, 3,804,479, 3,713,932, 3,700,295 and 3,616,000, bearing elements are disclosed having such a low friction, or self-lubricating, fabric material bonded to the inner surface of bearing elements. These patents generally disclose a process whereby fabric is applied over a mandrel and the structural body of the bearing element is built up over the fabric. Resin-impregnated fiberglass filaments are repeatedly wound about the fabric in order to form the body of the bearing element. Upon curing and hardening of the resin the fiberglass body becomes bonded to the fabric, the fabric thereafter forming the inner bearing surface. These known methods only produce bearing elements with low friction inner surfaces. These inner surfaces may additionally have irregular configurations, such as the surfaces found on the inside of the lined nuts disclosed in U.S. Pat. No. 3,713,932.
Bonding a low friction material to an existing outer or external surface of a finished bearing element to produce a long life bearing surface has been an unsolved problem for decades in spite of extensive development in this art. The low friction material, particularly self-lubricating fiber, by its nature resists bonding to another surface. Existing methods do not produce an external bearing surface that will resist pealing, galling and abrading in high stress load applications and known bearings cannot be used in such applications without unacceptably high frequent replacement. U.S. Pat. No. 5,087,132 discloses a method for lining the outer surface of a straight cylindrical bearing element, such as a pipe, with a low-friction fabric. The surface is pre-treated, in an attempt to provide a surface which will bond to the cured resin, the fabric and resin are degassed and the fabric wound over the surface with the resin. The resin-impregnated fabric is subsequently cured to affix the fabric to the surface.
Another approach for applying a low friction surface to a cylindrical outer surface comprises applying resin to such a surface, wrapping it with a film of high-wearing material, such as the polyester mylar, and heat shrinking the film to conform it to the surface. This approach has not been successfully utilized to apply a low friction surface to a spherical or otherwise irregularly configured outer surface. Mylar will not shrink to conform to recessed circumferential areas.
The problem is even more difficult, however, when the existing surface is an irregular-shaped, noncylindrical external surface, such as a spherical surface. It is very difficult to bond low friction fabric material over an irregular external surface. Problems arise in conforming the fabric to the outer surface in a tight fitting configuration, with difficulty encountered in thoroughly impregnating the cloth with the bonding resin. Upon cure of the resin there is no certainty in obtaining a secure mechanical interlock between the self-lubricating fibers of the cloth and the surface of the structural bearing element. U.S. Pat. No. 5,087,133 does not disclose forming a tightly toleranced bonding of an irregular-shaped substratum external surface to fabric.
U.S. Pat. No. 3,891,488 is one attempt to overcome this difficulty. This patent discloses a method of producing a bearing element by repeatedly winding a resin coated bondable low friction filament over an external spherical surface to form a low friction surface. The filament is first impregnated with resin and the resin partially cured to a tacky state in an effort to both make the filament adhere to the surface and to form a uniform single layer over that surface. The pre-treated filament is then wound on cones for handling prior to winding onto the bearing element surface. A single layer of thread is formed by helically wrapping the thread over the entire external surface. Heat and pressure are then applied in order to conform the fibers to the external surface, to finish the cure of resin and bond the low friction filaments to one another and to the surface. Roughening or similar such treatment to the surface of the bearing element is necessary to provide a good bond with the resin.
The known methods are complicated and costly, requiring various steps such as pre-coating and pre-partial curing of resin on low friction filaments, winding of filament, pressing and heat curing of the surfaces in dies, all designed to secure self-lubricating filaments to an external surface, and yet still do not produce a bearing surface that will have a long life in high stress load applications.